The present invention relates to bulk acoustic wave resonators, such as are used in providing bulk acoustic wave filters, and also to other devices built up from layers of thin film. More particularly, the present invention relates to providing a piezoelectric thin film, or a thin film of some other material, as a layer of such a resonator or as a layer of some other, similar device.
A thin film bulk acoustical wave (BAW) resonator is a structure consisting of various layers of different materials deposited one on top of the other, starting with a first layer deposited on a substrate which is typically silicon (Si), glass, gallium arsenide (GaAs), or silicon dioxide (SiO2). As shown in FIG. 1, one of the layers of a BAW resonator is a layer of piezoelectric material, also called a piezolayer, which gives the BAW resonator its characteristic resonance properties, making it useful as a component of a filter. The piezolayer is deposited on top of a layer of metallic material serving as one electrode, and then a second layer of metallic material is deposited on the piezolayer. Typical materials used for the piezolayer include zinc oxide (ZnO) and aluminum nitride (AlN).
The prior art teaches using magnetron sputtering to deposit a piezolayer on a layer of material (typically a layer of metallic material serving as an electrode), i.e. a substrate. The magnetron creates a plasma by accelerating electrons in a low pressure gas (of typically argon). A bias voltage is applied to the target (i.e. the sample of the material to be deposited, such as ZnO) so as to cause the surface exposed to the plasma to be at a negative potential, which causes the target surface to be bombarded by positive ions of the (low pressure) plasma (i.e. usually argon ions). The positive ions have very high energy and so vaporize atoms on the target surface. The vaporized atoms fly to the substrate and so form the thin film layer on the substrate.
The maximum bandwidth of a filter based on thin film resonators (FBAR) is determined by the effective piezoelectric coupling keff of the resonators. This is ultimately limited by the properties of the piezoelectric layer. The values for piezoelectric coefficient e of the materials typically used for a piezolayer, such as AlN and ZnO, are sufficiently high when the materials are in the form of bulk, single crystalline specimens, but in thin-film form the values attained are usually far below the bulk values because of the unfavorable microstructure of the thin film as produced by the present thin film deposition processes.
The important factors in determining the piezoelectricity of a thin film are crystallographic characteristics including crystallinity, crystal orientation, and grain size, but practice has shown that their significance is secondary compared to the state of stress of the thin film when deposited. Besides the crystallographic characteristics, a good piezoelectric thin-film material invariably exhibits a compressive stress. It appears from practice that having a compressive stress within some suitable range is the only indisputable criterion for an excellent piezofilm.
The primary parameters in adjusting the film stress in sputter deposition are the pressure of the sputtering atmosphere and the bias voltage applied to the substrate. Lowering the pressure or increasing the bias voltage shifts the stress in the compressive direction. Both of these factors increase the particle bombardment of the growing film. When a piezolayer is being deposited on an electrode layer that ultimately rests on a dielectric (glass) substrate, the capacitive bias voltage is ill-defined and its control imprecise, and the gas pressure remains the only practical control parameter. By altering the gas pressure, the piezolayer can be grown in either tensional or compressive stress. However, too high a stress (either compressional or tensional) will break the film or cause micro-cracks, and a film even with micro-cracks is not suitable piezoelectrically. A high enough stress will even delaminate a piezolayer from the bottom electrode. Therefore, a precise control of the stress state is essential, and such control is achieved principally through control of the gas pressure.
While it is quite straightforward to grow a ZnO film either in tension or in a high compressive stress state using magnetron sputtering, it has turned out to be difficult to achieve moderate values of compressive stress of the piezolayer, and it is moderate values of compressive stress that are needed for an acceptable piezolayer. Moderate values of compressive stress are difficult to achieve because magnetron sputter deposition at intermediate gas pressures is unstable. The reason for the instability is apparently due to the behavior of the plasma. Other parameters, besides the gas pressure, that determine the characteristics of the plasma are the gas composition, drive voltage, and history of the glow, the glow history being a factor because the plasma is non-linear (in its current-voltage characteristics) and may attain several steady-state glow modes that persist until something induces the plasma to switch to another mode. Perhaps because of the non-linear properties of the plasma, in practice, a deposition process using a gas pressure that would at least sometimes yield a compressive stress in a desirable range does not always do so; the process is not reliable, i.e. the results are not reproducible. Many of the resulting layers have a stress outside of the acceptable range, while others are nicely within the acceptable range. The yield of the process, i.e. the number of acceptable layers compared to the total number of layers produced, is, however, low enough to be a significant contributor to the cost of fabrication.
What is needed therefore, is a process of depositing piezoelectric material on a surface that consistently provides that the piezolayer has an appropriately moderate value of compressive stress, a value that will provide advantageous piezoelectric qualities.
Accordingly, the present invention provides a method and a device made according to the method, the method being of use in fabricating a device comprising a thin film of material, the fabrication using magnetron sputtering to deposit the thin film, the method comprising the steps of: a) determining a first gas pressure at which the magnetron sputtering is stable and results in the material being deposited having a first stress; b) determining a second gas pressure at which the sputtering is stable and results in the material being deposited having a second stress, the second stress having a value that is less than a desired intermediate stress; and c) performing successive sputtering cycles, each cycle including sputtering at the first gas pressure so as to achieve a predetermined first thickness, and sputtering at the second gas pressure so as to obtain a predetermined second thickness; thereby depositing a thin film of the material having an average stress intermediate between the first stress and the second stress.
In a further aspect of the invention, the material being laid down is a piezoelectric material.
In another, further aspect of the invention, the device is a bulk acoustic wave resonator.
In the usual application, the thin film is built up incrementally, using many successive cycles of sputtering at first the first gas pressure and then the second gas pressure.